As you say, there are plenty in the Northeastern United States. I think the primary historical factors that separates cohesive, walkable towns in the US from more stereotypical American development are the city having been established and built out prior to the 20th century (so there was some initial plan or development that predates cars), and the community either being wealthy enough or remote enough to avoid the pull of cookie-cutter post-war redevelopment. Of the places I’m familiar with, I’d call out Delaware and Montgomery County, PA, much of the Hudson Valley in New York (Hudson, Beacon, Newburgh, …). Outside of the northeast, there are remote locations (i.e. outside of commuting distance from a major city) in the southwest that have clearly defined and stroad-less cores. I’m thinking specifically of Taos County, NM and much of southwestern CO.

Ithaca is pushing into city territory (I think it has over 80k residents now), and if we’re going to count it, I’d also nominate small southeastern cities such as Savannah, Richmond, and St Augustine.